remote: Add extra parameter pkipath for URI

remote: Add extra parameter pkipath for URI

This new parameter allows user specifies where the client
cerficate, client key, CA certificate of x509 is, instead of
hardcoding it. If 'pkipath' is not specified, and the user
is not root, try to find files in $HOME/.pki/libvirt, as long
as one of client cerficate, client key, CA certificate can
not be found, use default global location (LIBVIRT_CACERT,
LIBVIRT_CLIENTCERT, LIBVIRT_CLIENTKEY, see
src/remote/remote_driver.h)

Example of use:

[root@Osier client]# virsh -c qemu+tls://10.66.93.111/system?pkipath=/tmp/pki/client
error: Cannot access CA certificate '/tmp/pki/client/cacert.pem': No such file
or directory
error: failed to connect to the hypervisor
[root@Osier client]# ls -l
total 24
-rwxrwxr-x. 1 root root 6424 Jan 24 21:35 a.out
-rw-r--r--. 1 root root 1245 Jan 23 19:04 clientcert.pem
-rw-r--r--. 1 root root  132 Jan 23 19:04 client.info
-rw-r--r--. 1 root root 1679 Jan 23 19:04 clientkey.pem

[root@Osier client]# cp /tmp/cacert.pem .
[root@Osier client]# virsh -c qemu+tls://10.66.93.111/system?pkipath=/tmp/pki/client
Welcome to virsh, the virtualization interactive terminal.

Type:  'help' for help with commands
'quit' to quit

virsh #

* src/remote/remote_driver.c: adds support for the new pkipath URI parameter